The present invention relates generally to telephone circuits and more specifically subscriber line interface circuits with reduced contact resistance.
Subscriber line interface circuits (SLIC) are well-known in the prior art. These circuits, which serve to connect the subscriber to a telephone exchange, typically include a current detector which performs several functions including detection of the subscriber going off-hook during ringing (ring-trip). In particular, when the subscriber is being called, AC ringing signals are applied to the subscriber loop to operate the telephone ringer. When the subscriber goes off-hook, the resulting loop current is detected to interrupt the ringing signals on the loop.
Another function performed by the SLIC is ringing and ring supervision. Ringing signals are generated in the SLIC and corresponding ring voltages are generated across the tip and ring terminals differentially. The SLIC includes a pair of line driving output amplifiers which drive the tip and ring lines respectively. As illustrated in FIG. 1, a SLIC 10 includes tip amplifier A1 and ring amplifier A2 driving a telephone exchange tip and ring line respectively through balancing resistors R1 and R2. The contact resistance of the connection of the tip and ring lines and their balancing resistors to the SLIC 10 is illustrated by contact resistance RC1 and RC2 respectively. The input or monitoring portion of the exchange tip and ring lines are provided from the balance resistors R1 and R2 to the SLIC 10 by separate connections.
The signal to be transmitted RX from the subscriber is received by the SLIC and coverted to a voltage VR.sub.x. The contact resistance RC1 and RC2 not only affect the balancing of the exchanger tip and ring lines, but also their imbalance affect the frequency response of the system. For example, 100 milliohm contact resistance differential could cause a shift and longitudinal balance reading of -63 dB to -52.1 dB on a -56 dB signal.
We have considered synthetic resistance devices or circuitries for compensating for contact impedance, but determined them to be difficult to calibrate and requiring increased real estate on the SLIC.
Thus, it is an object of the present invention to provide a SLIC with reduced contact resistance between the exchange tip and ring line and tip and ring line drivers.
Another object of the present invention is to provide a simple method of reducing contact resistance in a SLIC.
These and other objects are attained by providing separate and distinct contacts for the output of a tip and ring line drivers and their feedback resistors to the balance resistor of the appropriate tip and ring line. Thus, a Kelvin contact arrangement is produced providing a compensation for the contact resistance of a single connection.